The present invention relates to the machining of materials, and more particularly is a method and apparatus for increasing cutting tool life when machining materials with interruptions by cutting such as occurs in turning, facing, boring, milling, and drilling with cutting inserts.
Interruptions can be simple voids or portions of material with significantly different resistance to cutting. When interruptions exist in a material, the cutting force intermittently drops to a fraction of the maximum cutting force as the cutting edge passes over the interruption, e.g. to less than 50% of the maximum cutting force. (For the case of a void, the cutting force will drop to zero.) The size of the interruption must be large enough relative to the curvature of the cutting edge in the feed plane in order for the tool to experience an interruption in the force on the tool.
Interrupted cutting is a difficult machining operation, due to a shock-wave-type cyclic thermal and mechanical loading of the tool. Interruptions in workpieces act as impact stress-concentrators inducing premature fractures of the tool insert. The use of the conventional flooding or jetting cooling medium, applied “externally” to the cutting tool and/or in the general area of contact between the tool and the work material results in cooling and hardening of the edges of the material interruptions, leading to higher impact forces on the insert, and thereby leads to accelerated tool wear and failure, usually by fractures. The absence of cooling shortens the life of the cutting tool due to operation at high temperature involving thermal softening and chemical wear of the cutting edge as well as the rest of the insert. The net result is that tool life is drastically shorter during interrupted cutting relative to non-interrupted cutting whether the cooling is used or not. Most interrupted cutting operations are run dry to prevent cooling and hardening of interruption edges and at slow speeds to prevent thermal softening of the cutting tool.
As used herein, the term “cutting” includes but is not limited to the following operations: turning, boring, parting, grooving, facing, planing, and milling.
Related disclosures include U.S. Pat. Nos. 2,641,047, 3,077,802, 3,571,877, 3,971,114, 4,848,198, 5,237,894, 5,716,974, 5,901,623, 6,652,200 and U.S. patent applications Ser. Nos. 2002/0189413 and 2003/0110781. There is a need for a method that increases the tool life for interrupted cutting.